Page 453 - Fiber Bragg Gratings

430 Chapter 9 Measurement and Characterization of Gratings

mode coupling of light into the cladding, which can be detected easily
[39]. The writing process causes an asymmetry, which assists a directional
coupling to the cladding modes (see Chapter 4). Apart from the coherent
scattering mechanism, there is also in coherent scatter due to damage in
the core as in a Type II grating [40]. This type of grating scatters light
because of large surface irregularities at the core-cladding boundary.
A third type of scattering mechanism is incoherent and is due to the
inhomogeneity in refractive index modulation through the length of the
grating. The latter has been investigated by Janos et al. [41]. There ap-
pears to be a pronounced scatter out of the fiber perpendicular to the
direction of the writing beam. The observed anisotropy is consistent with
the production of "scattering elements" within a few microns of the core.
The scattering loss ranges from 0.2 dB/cm in highly doped Yb/Er phospho-
silicate fiber with gratings inscribed with a pulsed laser 193-nm source,
5
to 5 X 10~ dB/cm for a boron-codoped fiber with a CW 244-nm source.
A technique that measures the sideways diffraction from a fiber Bragg
grating is used to characterize the grating refractive index profile. A
schematic of this method is shown in Fig. 9.23. In this arrangement, light
from a He-Ne laser is focused form the side of the fiber, incident at an
angle <% to the normal. The schematic of the side-scatter measurement is

Figure 9.23: The side-diffraction scheme for characterizing Bragg gratings
(from: Krug P, Stolte R and Ulrich R, "Measurement of index modulation along
an optical fiber Bragg grating", Opt. Lett., 20(17), 1767-1769,1 September 1995.
[31]).